Build-up multilayered wiring boards are wiring boards developed for densely mounting various electronic components. The build-up multilayered wiring boards are used in various digital and/or mobile devices, and structured in such a manner that a plurality of wiring layers composed of copper wiring and resin and a plurality of resin base material layers composed of resin and fiber bundles are alternately laminated.
First, a typical build-up multilayered wiring board will be described. FIG. 12 illustrates a partial cross section of a build-up multilayered wiring board 100g (in some cases below, simply referred to as a “board”). In the board 100g, n (where n is an even number of 4 or more) wiring layers (C1 to Cn) and (n−1) resin base material layers [B1 to B(n−1)] are alternately laminated. Hereinafter, when collectively referring to each of the wiring layers and the resin base material layers, they will be indicated as “wiring layers C” and “resin base material layers B”, respectively.
The wiring layers C include copper wiring 101 and insulating resin 103. The resin base material layers B typically include plain-woven fiber bundles 102 impregnated with the insulating resin 103. Also, some of the wiring layers C (in the example shown, the wiring layers C1, C2, and Cn) include dummy wiring 108 to be described later. Note that FIG. 12 schematically illustrates the resin base material layers B with the fiber bundles 102 being impregnated with the resin 103. Similar illustrations are provided in other figures to be described later.
The fiber bundles 102 are typically glass fibers or aramid fibers. Also, the insulating resin 103 is thermosetting resin such as epoxy resin, phenol resin, polyimide, or BT resin (bismaleimide triazine resin).
Typically, the wiring layers C and the resin base material layers B are formed by alternately layering fiber bundles impregnated with insulating resin and copper foil having a wiring pattern formed thereon and curing the resin under pressure and heat (laminating press). Also, the resin 103 included in the wiring layers C is formed by a part of the resin in the impregnated fiber bundles entering gaps in the wiring pattern under pressure and heat.
Although not shown, the resin base material layers B have via holes or through holes formed therein, so that adjacent wiring layers C are electrically connected with each other via the via holes or the through holes. Note that the structure of such a build-up multilayered wiring board is defined in detail in the JPCA standard “Build-up Wiring Boards” (Standard No. JPCA-BU01-2007; see in particular Example Structures 3 and 4 on page 2).
The resin base material layers B are divided into a base layer 104, which is the central layer of the multilayered structure formed by laminating press as described above, and build-up layers 105, which are layered above and below the base layer 104. The same or different resin base materials may be used to form the base layer 104 and the build-up layers 105. On the other hand, the resin base materials having the same fiber bundle weave and content are used for the build-up layers 105.
In reflow soldering, the board 100g is placed on a reflow belt or pallet with electronic components being temporarily fixed on its top and bottom mounting surfaces. The board 100g is heated from room temperature to 220° C. or higher, and then cooled to room temperature after soldering. At this time, the board 100g might be warped due to the difference in the amount of thermal expansion between wiring layers, which are attributed to the difference in the copper remaining ratio (the area ratio of copper wiring in the entire wiring layer C) between the wiring layers. The mechanism in which the board is warped will be concretely described with reference to FIG. 13.
A board 100h shown in FIG. 13 has six wiring layers (from top, C1 to C6), and five resin base material layers provided between the wiring layers (from top, B1, B2 (build-up layers 105), B3 (base layer 104), and B4 and B5 (build-up layers 105)). The copper remaining ratios for the wiring layers are 32%, 28%, 37%, 46%, 52%, and 54% in order from the wiring layer C1. In this case, when the average copper remaining ratios are calculated for the layers (C1 to C3) overlying the base layer 104 (resin base material layer B3) and the underlying layers (C4 to C6), the average copper remaining ratio for the underlying layers is higher.
Comparison between the copper wiring 101 and the resin 103 included in the wiring layers C shows that the resin 103 has a higher linear expansion coefficient than the copper wiring 101. Accordingly, any wiring layer with a high copper remaining ratio has a lower amount of thermal expansion under temperature load. As a result, in the board 100h shown in FIG. 13, generally, the layers overlying the base layer 104 have a high amount of thermal expansion, and the underlying layers have a low amount of thermal expansion. Thus, when the temperature rises, the board 100h is warped convexly.
Mounting electronic components with the board being warped due to reflow soldering results in significantly reduced connection reliability between the electronic components and the board, which is a major factor for quality degradation of electronic circuits having the multilayered wiring board incorporated therein.
Conventionally, in order to prevent the board to be warped at the time of reflow soldering, the wiring layers are formed with the same copper remaining ratio to the greatest extent possible (see Japanese Laid-Open Patent Publication No. 2000-151035). Specifically, in order to minimize the difference in the amount of thermal expansion between the wiring layers resulting from the difference in the copper remaining ratio, dummy patterns are formed on the wiring layers in addition to the wiring to be included in the electronic circuit.